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Full text of "The Cambridge Natural History"

JURASSIC   MAMMALS

the family, is Plagiaulax. As it is not Triassic, the consideration
of its characters will be deferred until later. Microlestes is a
Rhaetic genus, loiowii from rocks in Germany and England ;
but it is entirely based upon molar teeth. M. antiques has a two-
rooted molar of an elongated form with a row of tubercles on either
side of a median groove, which traverses the long axis of the
tooth. To some extent the teeth of the ancient form resemble
those of OrnitJiorhyncJius. Microlcstes has been sometimes spoken
of as a Marsupial, but Mr. Tomes * has found that it does not
ahow one very universal character of the Marsupial teeth :
it has not those continuations of the dentinal tubes which
traverse the enamel in all Marsupials that have been examined
with the sole exception of the Wombat.
The rarity of the remains of mammals in these earliest rocks
of the Secondary epoch -has been accounted for in another way
from that which has been suggested above. It may be that the
group Mammalia was not evolved in Europe at all, and that the
stray remains which have been found in that continent represent
the fragmentary remnants of a few scattered immigrants which
heralded the later invasion of more numerous genera during the
Jurassic period.
The Mammals of the Jurassic Period.  Some of the Allo-
theria or Multituberculata described in the last section occur in
the rcfcks of this early part of the Secondary epoch. They are
doubtful in position, as already stated ; some of them indeed,
as for instance Tritylodon and Dromathcrium, are possibly not
mammals at all, while the remainder probably belong to a non-
existent order of mammals. Along with these dubious creatures
are the fragmentary remains of small animals which are not
merely mammals, but in all probability definitely Marsupials.
It is true that here again we have little beyond lower jaws
and teeth to deal with ; so that there may be less certainty
in referring them to the Marsupials than appears to be the
opinion of the majority of Palaeontologists.
Professor Osborn in fact considers that the Mesozoic mammals
consist of three groups : (1) The Multituberculata, including
the Bolodontidae, Stereognathidae, IMagiaulacidae, Polymasto-
dontidae, and possibly the Tritylodontidae (which, however, are
regarded by him and by others as more probably reptiles of the
1 Dental Anatomy, 5th ed. 1898, p, 304.